1. Technical Field
This invention relates generally to gas turbine engines and particularly to a flow passage thereof having reduced secondary flow losses therethrough.
2. Background Information
A gas turbine engine includes a turbine module with one or more turbines for extracting energy from a stream of working medium fluid. Each turbine has a hub capable of rotation about a longitudinal axis of the engine. The hub typically includes peripheral slots for holding one or more rows of blades. Each blade usually includes an attachment adapted to fit in one of the slots, a radially inner platform and an airfoil. When the blades are installed in the hub, the platforms cooperate with each other to partially define a radially inner boundary of an annular working medium flow duct. The airfoils span across the flow duct so that the airfoil tips are in close proximity to a nonrotatable casing. The casing circumscribes the blade row to partially define the radially outer boundary of the annular flow duct. However, the blades may also have radially outer platforms or shrouds that partially define the radially outer boundary of the annular flow duct.
A typical turbine module also includes one or more arrays of vanes that are nonrotatable about the engine axis for directing working medium flow through the turbine. Each vane may have radially inner and outer platforms that partially define the radially inner and outer annular flow duct boundaries. A vane airfoil spans across the flow duct from the inner platform to the outer platform. Thus, it will be seen that the annular flow duct comprises a multiplicity of flow passages defined by pairs of adjacent blade or vane airfoils, platforms and shrouds (or the engine case if the blades are shroudless). The platforms and shrouds or the adjacent portion of the engine case if the blades are shroudless, are typically referred to as passage endwalls.
During engine operation, a stream of working medium fluid flows through the above-described annular flow duct. Near the endwalls of the flow passages between pairs of adjacent vanes or blades, the working medium fluid flow may exhibit a phenomenon known as horseshoe vortices. Horseshoe vortices form as a result of the endwall boundary layer of the working medium fluid separating from the endwalls as the fluid approaches the leading edges of the airfoils. The separated flow reorganizes into horseshoe vortices as a result of mixing with the main fluid flow through the flow passage. There is a high loss of efficiency associated with such horseshoe vortices. This loss is referred to as a “secondary” or “endwall” loss. As much as 30% of a loss in efficiency in a row of airfoils can be attributed to endwall losses. While various schemes have been proposed in the prior art to reduce the losses associated with horseshoe vortices, such schemes have shown to be somewhat less than optimal, particularly when applied to turbines having low aspect ratio airfoils which do not provide a high degree of turning of the fluid flowing past the airfoils. Such low turning, low aspect ratio airfoils are employed in mid-turbine frame engine architectures in which the bearings for the low pressure and high pressure turbine rotors are mounted on a common frame structure disposed longitudinally between the low pressure and high pressure turbines. Accordingly, improved turbine flow passages wherein the formation of horseshoe vortices and the losses associated therewith are reduced, are sought.